The invention relates to a method for air-drying material webs, in particular material webs having a relatively high grammage, such as pulp webs. In the method, air blowings are applied in a direction substantially perpendicular to the web and in a direction substantially parallel to the plane of the web to be dried from underneath the web. The air blowings cause heat to be transferred to the web, supported the web by air free of contact, and stabilize the run of the web through the dryer.
The invention also relates to a nozzle-blow-box of an air dryer through which box air blowings are applied to the material web to be dried. By means of the air blowings, both the transfer of heat is produced from the drying air to the web and a contact-free air support and stabilization of the run of the web are obtained. The nozzle-blow-box comprises a box part having a nozzle-carrier face placed against the web. A substantially V-section groove is arranged transverse to the running direction of the web in the middle of the nozzle-carrier face. The groove is opened toward the web and has opposite walls in which a series of nozzle holes are arranged. Support and stabilization air blowings can be applied out of the series of nozzle holes such that they are crosswise and of opposite directions in relation to each other. Plane nozzle-carrier-face portions are placed in the same plane with each other at both sides of the V-section groove in the nozzle-carrier face.
The invention further relates to a pulp dryer that makes use of the method of the invention and/or the nozzle-blow-box of the invention.
In prior art through-dryers used in the paper and pulp industry, blow boxes are commonly used whose nozzle-carrier face consists of a plane plate in which blow holes have been punched. These nozzles are placed either at one side or at both sides of the airborne web to be dried. The nozzle-carrier-face commonly includes a number of rows of holes, one row after the other in the running direction of the web. The blow air flows in a space between the web and the nozzle-carrier-face, and the blow air is collected away through suction slots placed between the nozzle boxes.
In the prior art direct-blow nozzle boxes of air dryers for paper, board or pulp webs, wherein the air blowings are directed perpendicularly to the material web to be dried, a well-known problem is the lateral flow of the consumed air between the web to be dried and the nozzle-carrier-face. As used in the art, the term "lateral flow" is understood as meaning air flows parallel to the plane of the carrier face and of the web. These air flows are additionally parallel or opposite to the running direction of the web. Since the air must escape from the treatment gap, a lateral flow cannot be avoided. The lateral flow deteriorates the transfer of heat in the prior art blow-nozzle boxes, and a disturbing effect is increased with an increase in the velocity of the exhaust-air flow. Further, the loss of pressure produced by the blow box is increased when the velocity increases in the lateral flow.
On the other hand, in view of the runnability of the web to be dried, it is preferable to make use of the lateral flow by shaping the blow face in the blow box and the geometry of its nozzle openings such that a zone of negative pressure is formed on the carrier face of the blow box. This zone of negative pressure stabilizes the run of the web and ensures a stable and unstrained run of the web.
With respect to the prior art most closely related to the present invention, reference is made to the Swedish Patent No. SE 8,106,152 (corresponding to U.S. Pat. No. 4,505,053) and to International Patent Application No. WO 88/08950 (corresponding to U.S. Pat. No. 5,016,363).
In the blow boxes described in the Swedish patent, triangular openings, so-called "fish eyes", have been punched into the plane nozzle-carrier-face of the blow boxes. The front edge of the openings the front edge, i.e. the base of the triangle, has sharp edges. A sharp edge is not a major drawback as long as the amount of air discharged out of the nozzle is sufficient. However, at times, the amount of air received by the nozzle may be reduced considerably from the dimensioned value, for example, if the filters for drying air are blocked. In this case, the web starts to contact the nozzle face. It has been noticed that the sharp edges plane material out of the face of, e.g., a pulp web, in which case both the quality of the finished product is deteriorated and rubbish remains in the dryer.
Moreover, the rubbish remaining in the dryer disturbs the threading of the pulp web. In this regard, the formation of a "cigar" is spoken of, for the material detached out of the face of the pulp web by planing forms a roll resembling the structure of a cigar.
In the type of nozzle-blow-boxes employed in pulp dryers in which the web runs above the nozzle and carrier faces of the boxes, the function of the air blowings is both to transfer heat from the blown air to the web and to support the Web free of contact. In view of the runnability of the web, it is preferable to blow part of the air in a direction parallel to the plane of the nozzle such that the web is stabilized at a distance of about 3 mm to about 6 mm from the carrier face. However, in such a case, in the prior art nozzle-blow-boxes, the velocity of the exhaust air in the space between the web and the nozzle becomes high. This results in deterioration of the transfer of heat and in extra and/or excessive pressure losses. The detrimental effect of the high velocity of the exhaust air can be reduced by making the nozzles sufficiently narrow, but then the number of the nozzles becomes so high that the cost of manufacture of the dryer is increased substantially.